1. Field of the Invention
The present invention relates to a tilt control technique for controlling the tilting operation of masts of a fork lift truck, and more particularly to the flow rate control of a hydraulic control valve for controlling the operation of a tilt cylinder.
2. Description of the Related Art
In a conventional fork lift truck, there is provided a tilt cylinder control device comprising a forward tilting angle control function for putting limitations on the forward tilting angle of masts depending on lift height and/or load when the masts are tilted forward and a rearward tilting speed control function for slowing the rearward tilting speed of the masts when the masts are tilted rearward under a high lift height and high loaded condition.
These controls can be attained by providing on a passage connecting a rod side oil chamber of a tilt cylinder for tilting the mast with a tilt spool for operating the tilt cylinder a control valve capable of regulating the flow rate of hydraulic oil and of stopping a tilt cylinder at a predetermined position within its range of stroke.
A tilt control device comprising a control valve as described above is disclosed in, for example, Japanese Patent Laid-open No. 10-330096. In the tilt control valve disclosed in the unexamined patent publication, a proportional electromagnetic type flow rate control valve is provided on the aforesaid passage, and the proportional electromagnetic type flow rate control valve comprises a flow rate control part and a solenoid part for generating a pilot pressure, wherein the pilot pressure acting on the flow rate control part is controlled with an electromagnetic force (which is in proportion to current flowing through the coil) generated at the solenoid part, and wherein the displacement (opening) of a spool of the flow rate control part is determined by the balance between the pilot pressure and a spring force, whereby a flow rate in proportion to current is obtained.
With a proportional electromagnetic type control valve such as for use in the aforesaid tilt control device, however, there is a drawback that it is complicated in construction and is high in cost. In addition, with the proportional electromagnetic type control valve, the opening can be changed in a continuous fashion to thereby attain a fine control, but in contrast, there is a strong likelihood that a sticking phenomenon occurs.
The present invention has been made in view of the above problem, and an object thereof is to provide a tilt control technique for simplifying the construction of a tilt control device and for reducing cost for producing the same, while ensuring that the conventional masts forward tilting angle control function and rearward tilting speed control function are maintained.
According to the present invention, there is provided a tilt control device for a fork lift truck for allowing hydraulic oil received in a tank to flow into one of a rod side oil compartment and a bottom side oil compartments of a tilt cylinder using a hydraulic pump for tilting masts of the fork lift truck, the control device comprising:
a tilt spool for operating the tilt cylinder;
a pilot operation type flow rate control valve connected to the hydraulic pump via the tilt spool and adapted to be switched between a fully opened position and a half opened position which are different in opening from each other in response to addition/deletion of a pilot pressure;
a pilot operation type logic valve disposed between the rod side oil compartment of the tilt cylinder and the flow rate control valve and adapted to permit hydraulic oil to flow into the rod side oil compartment and to be operated so as to open/close relative to the hydraulic oil flowing out of the rod side oil compartment in response to the addition/deletion of the pilot pressure; and
an electromagnetic switching valve for controlling the addition/deletion of the pilot pressure to the flow rate control valve and the logic valve.
Consequently, according to the present invention, in tilting operation of the masts, when the electromagnetic switching valve for pilot operation is operated for switching, the pilot pressure is added or deleted relative to the flow rate control valve, whereby the flow rate control valve is switched to its fully opened position or half opened position. This switching of the flow rate control valve then leads to the control of the amount of hydraulic oil for the rod side oil compartment of the tilt cylinder in two ways; large and small, whereby the operating speed of the tilt cylinder is in turn switched between two speeds; high and low.
Thus, in operating the masts so as to tilt them rearward, the electromagnetic switching valve is operated for switching in response to respective detection signals representing rearward tilting operation, high lift height and high load, and the flow rate control valve is then switched from the fully opened position to the half opened position, thereby making it possible to control the rearward tilting speed of the masts.
In addition, in operating the masts so as to tilt them forward, when the electromagnetic switching valve is operated for switching, the pilot operation type changeover valve is changed over to a position where a pilot pressure is added or to another position where the pilot pressure is deleted relative to a poppet valve for an opening or closing operation, and this changing over of the positions of the changeover valve results in opening or closing of the poppet valve. In other words, the hydraulic oil in the rod side oil compartment is permitted to or prohibited from flowing out thereof by operating the electromagnetic switching valve for switching, whereby the tilt cylinder is put in operation or brought to a stop.
Thus, in forward tilting operation of the masts, the forward tilting angle can be controlled by switching the electromagnetic switching valve to a position to stop the operation of the tilt cylinder in response to detection signals representing forward tilting operation, high lift height and high load, and additionally a detection signal representing that the tilt angle of the masts reaches a given value.
As has been described heretofore, according to the present invention, functions equivalent to those provided by the conventional device can be secured through a combination of the ON/OFF type flow rate control valve, the changeover valve and the electromagnetic switching valve which are all low in cost and simple in construction, and moreover since the respective valves can be operated through a simple ON/OFF operation, there is less risk of generation of failure, thereby making it possible to improve the reliability.
Furthermore, according to the present invention, a fork lift truck can be provided which is fitted to the tilt control device having the specific features described above.